Microlens arrays are optical elements comprising a number of microlenses which are arranged in a single plane. It is expected that the demand for these arrays as principal optical elements will increase in the future in fine optics and other fields. Microlens arrays can, for example, be used in liquid crystal TV projectors. Each microlens in the array is arranged to correspond to a single pixel (or display region) of the liquid crystal panel. This particular application demands a microlens array which contains from twenty or thirty thousand to several hundred thousand microlenses in a single plane.
To optimize the efficiency with which the light is used, the microlenses are usually arranged in a dense pattern without spaces between them. However, such an arrangement gives rise to the following problems.
First, there is the problem of dust. The spaces surrounding the planoconvex microlenses are in effect steep-sided valleys in which dust readily accumulates. The dust which adheres to these areas cannot easily be removed with a cloth.
The second problem concerns the AR (anti-reflection) coating which is applied to the surface of each lens to minimize the reflective loss (Fresnel loss) from that surface. For this AR coating, an inorganic substance such as magnesium fluoride is used. This type of AR coating is built up on the surface of the lens by sputtering or deposition. However, the coating has a tendency to accumulate in the valleys between the lenses. As this happens, the AR film in the spaces between the lenses can become so thick that the surfaces of the lenses acquire flat spots.
The third problem concerns the production process. One method by which microlens arrays can be produced is electron beam drawing. However, this method is impractical, as it would take an extremely long time (from several weeks, say, to several months) to draw from twenty or thirty thousand to several hundred thousand microlenses with a diameter of 100 micrometers.
The fourth problem concerns microlense arrays made with round based individual microlenses. If round based microlenses are arranged in a single plane, it is unavoidable that there be regions between the lenses which do not function as lenses, even if the lenses are touching each other. The best effective less ratio which can be achieved when round microlenses are arranged in an array is 80%.
Since the curvature of the lenses in microlens arrays is microscopic, it is difficult to produce them by machining or polishing processes. Stamping is generally believed to be the optimal method for mass-producing microlens arrays. The stamping mold can be produced either by a machining process or by electroforming.
In the machining process, the shapes of the planoconvex microlenses are formed with sharp pointed tools. The portions corresponding to the valleys between individual microlenses are machined as steep-sided protrusions. Since these protrusions are microscopic, the sharp pointed can easily become rounded or damaged during the machining process.
If the stamping mold is electroformed, the surface of a master is made conductive by non-electrolytic plating using a catalyst. The material of the stamping mold is then built up by electrolytic plating, and the completed stamping mold is peeled off the master.
During the electrolytic plating used in this process, the so-called edge effect occurs in the valleys between the lenses, and less material will accumulate in the valley portions than on the other portions, or none at all will be deposited in the valleys. This effect results in the stamping mold not having the desired shape or strength required for manufacturing.